In many types of surgery, systems and devices are used to provide a surgeon with real-time positional guidance to guide therapy. For example, in THA, there exist systems that provide a surgeon with positional guidance of an acetabular implant with respect to a pelvis of a patient. Positional measurements are measured using tracking systems, typically utilizing optical, electro-magnetic, inertial, ultrasonic or RF measurement modalities.
Optical tracking systems, such as the Polaris™ system, manufactured by Northern Digital Inc. (Waterloo, ON), utilize fixed multi-camera arrays located in an operating room to detect targets within a working volume of the cameras. The targets normally have optically identifiable markers that are easily identifiable in a video feed of the cameras. Examples of markers include active or reflective markers in the infra-red frequency spectrum, markers of a distinct colour, markers with distinct shapes or patterns, which are easily positively identifiable in an image (e.g. checker pattern) etc. Commonly, reflective spheres are used as markers, since their centroids are well-defined and accurate; furthermore, spheres can be viewed from multiple angles. The markers and the cameras may have matched frequencies to filter unwanted light, either in hardware or in software.
Targets are normally comprised of a plurality of markers. A single marker provides an identifiable feature to the optical tracking system in the form of positional information. A plurality of markers associated with a single target would allow the pose (position and orientation) in up to 6 degrees of freedom (DOF) to be calculated by a computing or processing unit embedded within, or in communication with the optical tracking system. Each marker provides a well-defined and accurate feature. For medical and/or surgical applications, optical tracking systems are generally used to determine the pose of targets and the objects to which targets are attached (e.g. surgical instruments or tools, a patient's anatomy, etc.). Targets (comprised of a plurality of markers) must be registered (also referred to as calibrated) to the instrument to which they are attached; furthermore, they must maintain the registration throughout use.